The field of the invention is plastic components and, more particularly, the invention relates to automotive parts made of thermoset resin and reinforcing fiber mixtures laminated with in-mold paint sheets.
A modem automobile is manufactured from a large number of parts. Each of the parts that make up an automobile typically has a number of desired properties that are distinct to a particular part. In a particular automobile part, it may be more or less desirable for the part to be lighter or heavier, more rigid or flexible, or be in need of further finishing or not. Light weight, high strength, and good appearance (for example, as defined in the automotive industry, a “class A” surface) are generally desirable characteristics in auto parts because such properties tend to make automobiles more energy efficient, safer, and more attractive to consumers. Further, the issue of expense is always of great import. One example of the tradeoffs inherent in choosing how to make a part is the early auto industry's use of metal parts, such as steel parts, which were predominant through the 1970s. Although metal parts are strong and readily paintable to yield class A surfaces, they are also heavy and expensive to protect from corrosion and painting operations.
Various thermoset polymers (as opposed to thermoplastics) can be used to produce parts including parts for the manufacture of automobiles. One thermoset manufacturing technique uses sheet molding compound (SMC) (specific gravity of about 1.9 g/cc) which are generally lighter in weight than metal parts (specific gravity of about 7.8 g/cc). However, for the manufacture of certain parts, thermoset plastic parts, such as those made from SMC, cannot readily have necessary structures efficiently molded into them and further, even if formed cannot be inexpensively provided with Class A finishes. For example, in order to achieve a class A finish, a part made from sheet molding compound must be sanded, have a primer coat applied and dried, and then a base coat applied and dried, and a clear coat applied and dried. Further, each painting step may either require paints low in volatile organic compounds (which are expensive) or present significant costs and regulatory burdens because of the environmental impact of the painting operations. Last, SMC is always a two-piece assembly, not a one-piece assembly.
While some of the structural limitations of SMC parts can be addressed by gluing metal supports onto or between SMC components, SMC parts are not particularly amenable to a “one shot” solution in which a part having significant structural reinforcement and good appearance can be produced in one cycle of the compression mold. For example, rather than mold a steel support into an SMC part, a steel reinforcing sheet may be sandwiched between two sheets of SMC compound. However, achieving the result in that way generally uses two molding steps and one gluing step. Further, SMC does not expand in a compression mold, and is therefore subject to forming sinks. Such sinks should be repaired, if possible, before using the part. Because SMC is a thermoset plastic, and therefore cannot be meaningfully recycled, flaws that cannot be repaired are expensive and are environmentally unfriendly.
Some exterior auto and truck parts such as fenders, quarter panels, doors, trim parts bumpers, fender extensions and other molded exterior parts of are currently made of moldable thermoplastic polymers to obtain resistance to permanent deformation on impact and corrosion resistance. Typical moldable thermoplastic polymers that are being used for these parts are polypropylene, styrenics, and various heteroatom polymers. While such polymers can be molded into auto and truck parts that are substantially lighter than currently used sheet metal parts, but have limitations. In particular, such parts ordinarily do not pass through industry-standard painting operations with acceptable strength and durability. Further, such molded plastic parts have molded-in stress that releases when the part is exposed to heat, such as in a painting operation, thereby altering the dimensions or shape of the part. The release of the molded-in stress leads to unacceptable fit and finish of parts to other parts, and can lead to unacceptable deviations in the alignment of connected parts. Also, the reinforced resins that must be used to attempt to overcome the limitations of thermoplastic parts are quite expensive.
It would be advantageous to provide strong, finished parts that can be easily formed in one shot. It would be advantageous if such parts could be formed complete with reinforcing members, such as steel members, without adversely affecting the appearance of the part. It would be also advantageous if such results could be achieved by molding reinforcing members into the part without unduly lengthening cycling times for the manufacture of parts. Therefore, there is a need for parts that exhibit low weight, high strength and good appearance, and can be efficiently and cost-effectively manufactured.
A number of patents relate to various aspects of motor vehicle component technology. For example, U.S. Pat. No. 5,858,146 to Söchtig et al. discloses a process and device for manufacturing plastic parts with reinforcement fibers. The patent discloses that long reinforcing fibers can be incorporated in a flowable synthetic resin in a single working step and by a single apparatus and then introduced into a plastic mold, whereby per operating steps the lengths of the fibers and the density with which the fibers are incorporated can be varied in an optional manner. As will be recognized by those of ordinary skill, the apparatus disclosed in the '146 patent would present a fire and or explosion hazard if operated with ignitable fibers, such as carbon fiber. In addition, as will be recognized by those of ordinary skill in the art, the '146 patent discloses cutting machinery suitable only for fragile, brittle fibers such as fiber glass. The '146 patent is incorporated by reference into this application as if fully set forth herein.